Clive McCann

Relationships among compressional wave attenuation, porosity, clay content, and permeability in sandstones

Anelastic attenuation is the process by which rocks convert compressional waves into heat and thereby modify the amplitude and phase of the waves. Understanding the causes of compressional wave attenuation is important in the acquisition, processing, and interpretation of high‐resolution seismic data, and in deducing the physical properties of rocks from seismic data. We have measured the attenuation coefficients of compressional waves in 42 sandstones at a confining pressure of 40 MPa (equivalent to a depth of burial of about 1.5 km) in a frequency range from 0.5 to 1.5 MHz. The compressional wave measurements were made using a pulse‐echo method in which the sample (5 cm diameter, 1.8 cm to 3.5 cm long) was sandwiched between perspex (lucite) buffer rods inside the high‐pressure rig. The attenuation of the sample was estimated from the logarithmic spectral ratio of the signals (corrected for beam spreading) reflected from the top and base of the sample. The results show that for these samples, compressio...

Seismic attenuation and pore-fluid viscosity in clay-rich reservoir sandstones

The frequency dependence of seismic attenuation in a suite of clay‐rich reservoir sandstones was investigated in the laboratory. Compressional‐ and shear‐wave velocities (VP and VS) and quality factors (QP and QS) were measured as functions of pore‐fluid viscosity at an effective pressure of 50 MPa and at an experimental frequency of about 0.8 MHz using the pulse‐echo technique. The experimental viscosity ranged from 0.3 to 1000 centipoise, which gives equivalent frequencies for a water‐saturated sandstone of 2.6 MHz to 780 Hz, assuming a global‐flow loss mechanism. Two types of behavior were observed: high permeability (greater than 100 millidarcies) sandstones tend to show variable QP and QS which are similar in magnitude to those predicted by the Biot theory over the viscosity range 0.3 to about 20 centipoise (equivalent frequency range 2.6 MHz to about 39 kHz); low permeability (less than 50 millidarcies) sandstones tend to show almost constant QP and QS over the experimental viscosity range that are ...

Why is the Biot slow compressional wave not observed in real rocks

A general theory for the propagation of elastic waves in porous, fluid-filled media was derived by Biot (1956a, b). A fundamental prediction of this theory is the existence of three body waves: a shear wave, a fast compressional wave, and a slow compressional wave.

Experimental measurements of seismic attenuation in microfractured sedimentary rock

Ultrasonic compressional- and shear-wave attenuation in water-saturated Carrara Marble increase with increasing crack density and decreasing effective pressure. Between 0.4 and 1.0 MHz, empirical linear relationships between 1/Q and crack density CD were found to be:CD = 1.96 + or - 0.63 X 1/Q,for compressional waves andCD = 6.7 + or - 1.5 X 1/Q,for shear waves.

Sonic to ultrasonic Q of sandstones and limestones: Laboratory measurements at in situ pressures

Laboratory measurements of the attenuation and velocity dispersion of compressional and shear waves at appropriate frequencies, pressures, and temperatures can aid interpretation of seismic and well-log surveys as well as indicate absorption mechanisms in rocks. Construction and calibration of resonant-bar equipment was used to measure velocities and attenuations of standing shear and extensional waves in copper-jacketed right cylinders of rocks (Formula in length, Formula in diameter) in the sonic frequency range and at differential pressures up to Formula. We also measured ultrasonic velocities and attenuations of compressional and shear waves in Formula-diameter samples of the rocks at identical pressures. Extensional-mode velocities determined from the resonant bar are systematically too low, yielding unreliable Poissons ratios. Poissons ratios determined from the ultrasonic data are frequency corrected and used to calculate thesonic-frequency compressional-wave velocities and attenuations from the shear- and extensional-mode data. We calculate the bulk-modulus loss. The accuracies of attenuation data (expressed as Formula, where Q is the quality factor) are Formula for compressional and shear waves at ultrasonic frequency, Formula for shear waves, and Formula for compressional waves at sonic frequency. Example sonic-frequency data show that the energy absorption in a limestone is small (Formula greater than 200 and stress independent) and is primarily due to poroelasticity, whereas that in the two sandstones is variable in magnitude (Formula ranges from less than 50 to greater than 300, at reservoir pressures) and arises from a combination of poroelasticity and viscoelasticity. A graph of compressional-wave attenuation versus compressional-wave velocity at reservoir pressures differentiates high-permeability (Formula, Formula) brine-saturated sandstones from low-permeability (Formula, Formula) sandstones and shales.

The effects of pore‐fluid salinity on ultrasonic wave propagation in sandstones

Petrophysical interpretation of increasingly refined seismic data from subsurface formations requires a more fundamental understanding of seismic wave propagation in sedimentary rocks. We consider the variation of ultrasonic wave velocity and attenuation in sandstones with pore‐fluid salinity and show that wave propagation is modified in proportion to the clay content of the rock and the salinity of the pore fluid. Using an ultrasonic pulse reflection technique (590–890 kHz), we have measured the P-wave and S-wave velocities and attenuations of 15 saturated sandstones with variable effective pressure (5–60 MPa) and pore‐fluid salinity (0.0–3.4 M). In clean sandstones, there was close agreement between experimental and Biot model values of dVP/dM, but they diverged progressively in rocks containing more than 5% clay. However, this effect is small: VP changed by only 0.6% per molar change in salinity for a rock with a clay content of 29%. The variation of VS with brine molarity exhibited Biot behavior in so...

A new laboratory technique for determining the compressional wave properties of marine sediments at sonic frequencies and in situ pressures

We describe a new laboratory technique for measuring the compressional wave velocity and attenuation of jacketed samples of unconsolidated marine sediments within the acoustic (sonic) frequency range 1–10 kHz and at elevated differential (confining – pore) pressures up to 2.413 MPa (350 psi). The method is particularly well suited to attenuation studies because the large sample length (up to 0.6 m long, diameter 0.069 m) is equivalent to about one wavelength, thus giving representative bulk values for heterogeneous samples. Placing a sediment sample in a water-filled, thick-walled, stainless steel Pulse Tube causes the spectrum of a broadband acoustic pulse to be modified into a decaying series of maxima and minima, from which the Stoneley and compressional wave, velocity and attenuation of the sample can be determined. Experiments show that PVC and copper jackets have a negligible effect on the measured values of sediment velocity and attenuation, which are accurate to better than ± 1.5% for velocity and up to ± 5% for attenuation. Pulse Tube velocity and attenuation values for sand and silty-clay samples agree well with published data for similar sediments, adjusted for pressure, temperature, salinity and frequency using standard equations. Attenuation in sand decreases with pressure to small values below Q?1 = 0.01 (Q greater than 100) for differential pressures over 1.5 MPa, equivalent to sub-seafloor depths of about 150 m. By contrast, attenuation in silty clay shows little pressure dependence and intermediate Q?1 values between 0.0206–0.0235 (Q = 49–43). The attenuation results fill a notable gap in the grain size range of published data sets. Overall, we show that the Pulse Tube method gives reliable acoustic velocity and attenuation results for typical marine sediments.

Least-squares inversion of in-situ sonic Q measurements: Stability and resolution

We minimize the effect of noise and increase both the reliability and the resolution of attenuation estimates obtained from multireceiver full‐waveform sonics. Multiple measurements of effective attenuation were generated from full‐waveform sonic data recorded by an eight‐receiver sonic tool in a gas‐bearing sandstone reservoir using two independent techniques: the logarithmic spectral ratio (LSR) and the instantaneous frequency (IF) method. After rejecting unstable estimates [receiver separation <2 ft (0.61 m)], least‐squares inversion was used to combine the multiple estimates into high‐resolution attenuation logs. The procedure was applied to raw attenuation data obtained with both the LSR and IF methods, and the resulting logs showed that the attenuation estimates obtained for the maximum receiver separation of 3.5 ft (1.07 m) provide a smoothed approximation of the high‐resolution measurements. The approximation is better for the IF method, with the normalized crosscorrelation factor between the low‐...

The Effect of Degree of Saturation On Ultrasonic Velocity And Attenuation In Sandstones

Ultrasonic measurements of P-wave and S-wave velocity and quality factors (Qp, Qs) have been obtained from 9 sandstones, over a wide range of water saturations and confining pressures. The sandstones display a wide rang of porosity and permeability. Saturation state was varied in the sandstones by equilibrating the cores with various capillary pressures. Confining pressure varied from 5MPa to 60MPa. The values of the measured acoustic parameters were normalised to the fully saturated values to allow inter-sample comparison.

Effective stress coefficient for P- and S-wave velocity and quality factor in sandstone, Example from Cooper Basin-Australia

Summary The velocities and quality factors of compressional and shear waves have been measured at ultrasonic frequencies in Cooper Basin sandstone under a rang of pore pressure and confining stress using a pulse-echo technique. The effective stress coefficient, n, is found to be less than 1 for both velocities and quality factors in the Cooper Basin sandstone. The pore pressure sensitivity of wave velocities decrease ( n approaches one) with the increase of differential pressure. In contrast, the effect of pore pressure on both P- and S-wave quality factors increase ( n decreases) with increasing differential pressure. The strong influence of pore pressure on Q p and Qs is attributed to behavior of pore filling and grain coating clay minerals.